(a) Field of the Invention
The present invention relates to an objective lens system for endoscopes, and more specifically to an objective lens system so adapted as to perform focusing and at the same time adjustment of magnification to a level different from the ordinary magnification level matched with the distance to an object to be observed.
(b) Description of the Prior Art
For observation through an endoscope, the following functions are desirable for the observer: For observing an object located at a long distance, the endoscope should desirably permit the observer to observe a sufficiently wide range so as to complete observation in a short time without looking over abnormal portion. Further, the endoscope should desirably enable the observer to judge the location of the portion he is observing. For observing an object located at a short distance, on the other hand, the endoscope should desirably permit the observer to observe it at a sufficient magnification level for observing details of abnormal portion at a magnified scale and to provide sufficiently magnified even for locations near which the fiber scope cannot be brought.
In order to satisfy the desire for observing an object located at a long distance, among the two desires mentioned above, the endoscope requires an objective lens system having a short focal length, whereas it requires an objective lens system having a long focal length to meet the desire for observing an object located at a short distance.
Since it is very difficult to satisfy these two contradictrory desires at the same time, there have heretofore been known no objective lens systems for endoscopes that can satisfy the two desires at the same time. Though it is possible to satisfy the two desires at the same time by adopting a zoom lens system, it requires zooming operation and focusing operation which must be performed separately. In case of an endoscope which cannot be kept in fixed condition, however, it is difficult for the observer to perform both the zooming operation and focusing operation at the same time. Moreover, it is difficult from the technological viewpoint to arrange mechanisms for zooming and focusing respectively within a thin endoscope.